Dexta charging light

timff

Member
Hi all. In the past, the old girl's red light has stayed on until she'd run a few minutes at some revs but now it's staying on. Before I buy a new/recon dynamo, is there anything else I should check? The two wires are connected to the dynamo okay. The present dynamo is at least 40 years old so it's probably worn out? I inherited the old girl from Dad who bought her new in 1958!
If you reckon it's the dynamo, rather than buy a new/recon, is it worth getting it repaired by a specialist?
I'll be off air till Monday so many thanks in advance for any replies!
 

timff

Member
Thanks David. I sparked the control box yesterday but no change. I reckon at 60 years old its either the control box or dynamo so I'm awaiting a call from a chap who knows....
 
Change both
Oh the days of sorting out dynamos. I used to know how to set those lucas regulator boxes with my eyes shut and which terminal went where etc, but now I cant hardly even remember what those relay bobins and contacts under the aluminium cover even look like its been that long. :(
 
Polarise your dynamo 1st check your earth's too
If you remove the wires you can test the output of the dynamo or remove belt and short circuit it it should turn slowly like a motor this means the dynamo is functioning

If you do swap try and fit in pairs

But I would polarise the old one 1st

And you have to polarise the new one also
And make sure it's negative earth too
 
Polarise your dynamo 1st check your earth's too
If you remove the wires you can test the output of the dynamo or remove belt and short circuit it it should turn slowly like a motor this means the dynamo is functioning

If you do swap try and fit in pairs

But I would polarise the old one 1st

And you have to polarise the new one also
And make sure it's negative earth too
We're they sent out positive earth?
 

TheTallGuy

Member
Location
Cambridgeshire
Why were the old dynamo systems set up with pos earth?
It was largely a hangover from the early days of telegraph where the standard was to have positive earth/negative signal as it was easier to manage the galvanic corrosion on the earth circuit that way around. It wasn't until silicon diodes & transistors became viable & cheap that negative chassis became the standard system & some early alternators were positive chassis!

Strictly speaking, cars & tractors don't have "earth"/"ground" - it's "chassis" as there's no direct connection to the general mass of earth.
 
We're they sent out positive earth?
The new dynamos are neg earth only,
If the tractor is attached as pos earth then it won't charge and polarising won't have any effect

Unsure if dextas are neg or pos earth from new as most people like to bigger about with things and I've not done much with dextas if honest either

Had 1 dealing with one we restored and it kept putting the light on dim at tickover adjusting the reg box would sort this but it would cutout higher up the rev range,
Adjusting the ticking a 100rpm cured the issue after altering the reg box back to standard
 

Mursal

Member
And the manufacturers moved from positive "earth" because you always have a chemical reaction on the positive terminal of the battery from charging. This reaction caused corrosion where the positive "earth" bolted to the chassis. The negative terminal doesn't suffer from this .......
 

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